pH, or more generally the concentration of free Hydrogen atoms, is an important parameter of tissue such as skin. pH at the skin surface differs from individual to individual and is moreover dependent on external influences such as the application of cleansing products or other personal care products, such as deodorants. Cleansing products (water, soaps, shampoos etc) can have a pronounced effect on skin pH which only slowly returns to the pH-value before cleaning. In addition there is a large difference between pH of the surface of skin and the pH of deeper layers of the skin, because a large difference in pH exists between the skin surface and the vital epidermis. This pH difference is maintained by the stratum corneum which is the outermost skin layer. For the development of such products it would be desirable to monitor their effects on the pH of the skin.
Known methods of measuring pH involve for example the application of chemical pH indicators or electrical measurements, wherein the potential difference between an electrode on the skin and a reference electrode is measured. An article by K. I. Mullen, D. Wang, L. G. Crane, and K. T. Carron, titled “Determination of pH: SERS Fiber Optic probes”, published in Analytical Chemistry 64, page 930 (1992) describes a technique for measuring pH in water using a pH indicator molecule attached to the end of an optical fiber. Raman scattered light from the molecule is gathered through the fiber and analysed. With these presently available methods non-invasive skin pH measurements with the known methods are limited to the skin surface. Such measurements are unsatisfactory because of the large difference between pH of the surface of skin and the pH of deeper layers of the skin. Invasive techniques are needed to measure the pH below the surface of the skin with these known methods, or more generally to measure pH as a function of depth in the skin. The disadvantage of invasive techniques is that the pH may be affected by the invasive technique and/or that otherwise skin physiology is disturbed. Moreover it is difficult to monitor ongoing changes in pH.
Accordingly, there exists a need for a method of measuring pH at selected depths in skin tissue without using invasive techniques, so that the normal chemical processes in the skin are not influenced by the measurements.
An article titled “In Vivo Confocal Raman microspectroscopy of the Skin: Non-invasive Determination of Molecular Concentration Profiles” published in March 2001 in the Journal of Investigative Dermatology 116 pages 434-442 (2001), and authored by Peter J. Caspers, Gerald W. Lucassen, Elizabeth Carter, Hajo Bruining and Gerwin J. Puppels describes depth selective Raman spectroscopy of skin tissue. This article is incorporated herein by way of reference.
Raman spectroscopy is a non-invasive technique that involves illuminating material with essentially monochromatic light of a first wavelength and observing the intensity of light that has been inelastically scattered by the material, as a function of wavelength of the inelastically scattered light. The spectrum of the inelastically scattered light is a composite of contributions of the different chemical species in the material.
In principle, each chemical species provides its own characteristic contribution to the spectrum, in proportion to its concentration. This makes it possible to determine at least the relative concentrations of chemical species in the material. Raman Spectroscopy can be made depth and/or location sensitive by focussing the monochromatic light at a certain depth or location and/or gathering inelastically scattered light selectively from a depth or location.
Unfortunately, free protons do not have a measurable Raman spectrum. Therefore Raman spectroscopy cannot be used to measure light that is inelastically scattered by protons. However, the article mentions a discovery by the inventors of the present invention that among a great many other contributions the Raman spectrum of light scattered by skin tissue naturally contains a detectable contribution of UCA (Urocanic acid). The concentration of UCA in the stratum corneum depends on many variable factors. In addition the protonation of UCA varies significantly in the pH range that may be encountered in the skin (pH's typically range from 4.5 to 7). The Raman scattering spectrum of UCA is pH dependent.